The example provided is an argument from ignorance, although there are a variety of other fallacies that can also apply, as fallacies tend to bleed into each other, and people often commit multiple fallacies at the same time. It is not presented as an argument explicitly, as is the case in most non-formal discussion, but what Yvonne is implicitly saying is:
- I have a speculation A about the reason for X
- You have presented no alternative explanations, or evidence against A
- Therefore A must be true.
The assertion that 2) leads to 3) is a classic argument from ignorance.
It ALSO can be characterized as a shifting of the burden of "proof" (we can almost never "prove" things, this should be renamed the "burden of support").
Also, hidden in Yvonne's thinking is likely to be a false dichotomy, where A or B must be the explanation, and if no B can be identified or supported, then A must be the explanation, no matter how weak its support may be. Yvonne likely has some support or justification in her head, which she has not presented, which make her think that A is plausible.
Note, false dichotomy thinking is to some extent reinforced by someone getting trained in classical logic, where only two states, true or false, can apply to a claim. We have realized that for most questions, we need at least three logic states -- true, false, and uncertain. An empiricism operates off a four state logic, of "sufficiently supported", "sufficiently contradicted", "currently indeterminate vs support or contradiction", and "poorly formed claim, cannot be evaluated". Whether the universe was caused or not, and if so what caused it, is an empirical question, and empiricism's 4 state logic applies to it.
There are other ways one may "exploit a gap in the other person's knowledge", and that is to argue in bad faith. If one knows that an evidence or argument has factual or fallacious errors to it, BUT that one's disputant does not realize this, and presents those evidences or facts anyway, that is to argue in bad faith. Philosophy SHOULD be a search for truth, not an effort to convince of a conclusion. One of the most notorious examples of bad faith argumentation is to quote-mine -- taking a quote out of context that implies the speaker supports a conclusion, fact, or POV that is very different form their actual views. Quote-miners know they are presenting false claims in bad faith, and unless the disputant know the full context of the quote, they are in a position of ignorance about the bad faith behavior of the quote-miner.
As a significant side note there are limits to the application of "fallacy" labeling to arguments and justifications. ALL justifications for a POV will eventually run afoul of the Munchausen Trilemma. The chain of justifications for any justification will eventually terminate in one of three "fallacies". Unless one wants to abandon the ability to reason and acquire knowledge, then one must have tolerance for a degree of insufficient justifications.